A schematic view of a rectifier used in the power supplies of, inter alia, most personal computers is shown in FIG. 1 of the accompanying drawings. The rectifier comprises a connection to a power source (for example, the mains) and a connection to a load, i.e., the power drawn by the machine of which the rectifier forms a part. The power source and the load are connected to one another by a diode rectifier bridge, which allows current to flow only from the power source to the load, and not in the opposite direction. Two bulk capacitors, connected in series with one another, are connected in parallel with the load.
A (usually mechanical) voltage selector switch is connected between the two bulk capacitors and an input of the diode rectifier bridge. The rectifier may be switched between “standard rectifier mode” (for use with, for instance, 230V mains power sources as used in the UK) and “voltage doubler mode” (for use with, for example, Japanese 100V mains power sources).
While this rectifier is flexible and simple to build, it suffers from certain drawbacks. Due to the fact that the power source is connected to the bulk capacitors and to the load by a forward-biased diode rectifier bridge, current will only flow from the power source to the bulk capacitors and the load when the power source voltage exceeds that across the bulk capacitors. At other times, no current will flow from the power source. As a result, the rectifier draws an inherently non-sinusoidal current from the power source, and this introduces current harmonics into the power source. The introduction of such harmonics is undesirable as it can lead to a greater root mean square (i.e. heating) current in the rectifier, and can cause protection equipment to trip at lower power ratings than would usually occur. This problem is exacerbated if many users connect to the same power source and all introduce current harmonics into the power source.
The location of an inductor (known as a passive power factor correction (PFC) inductor) between the diode rectifier bridge and the bulk capacitors has the effect of reducing the amplitude of the current harmonics introduced into the power source. However, it is desirable to introduce an inductor of minimal size and weight into the rectifier circuit, to reduce the manufacturing and transportation costs of the rectifier.
In order for a passive PFC inductor to be of minimal size and weight and also be effective in reducing current harmonics over a range of input voltages, it is necessary for the inductor to have an inductance that varies with the current flowing therethrough.
However, it can be difficult to predict the inductance-current relationship that a passive PFC inductor must have to comply with a given regulation concerning minimum current harmonic introduction. Previous methods of predicting the required relationship have involved the correlation of data from multiple simulation runs to produce empirical design equations, such as those produced by Red1. When generating these equations, Red1 assumed that the bulk capacitors were sufficiently large that the ripple voltage across them (arising from the continual charging and discharging thereof) could be neglected. Apart from the large amount of time required to generate the empirical equations, a disadvantage of this approach is that in many practical situations the ripple voltage on the bulk capacitors is of sufficient magnitude that it cannot be neglected.